466 THE QUANTUM THEORYForever and in vain, Einstein kept looking for hints that would help him realize
his vision of a quantum theory derived from a unified field theory. This urge
explains his reference to the quantum theory at unexpected places. His first paper
with Grommer (see Section 15f) on the problem of motion ends, 'It has been
shown for the first time that a field theory can contain a theory of the mechanical
properties of discontinuities. This may become of significance for... the quantum
theory' [E28]. However, in a sequel he withdrew this last remark [E29]. In 1930
he gave a lecture on unified field theory, a report of which contains the statement,
'He emphasized that he is in no way taking notice of the results of quantum cal-
culations because he believes that by dealing with microscopic phenomena these
will come out by themselves' [E30]. A report in 1931 by Einstein on a five-dimen-
sional theory which should unify gravitation and electromagnetism ends, 'This
theory does not yet contain the conclusions of the quantum theory' [E31]. Two
months after the Einstein-Podolsky-Rosen article, Einstein and Rosen completed
another paper, this one dealing with singularity-free solutions of the gravitational-
electromagnetic field equations [E32]. One phrase in this paper, 'one does not see
a priori whether the theory contains the quantum phenomena' illustrates once
again the scope of the program that was on Einstein's mind. The program was to
remain an elusive vision. Gravitation and electromagnetism were not synthesized,
quantum physics was not integrated, satisfactory particle-like solutions were not
found.
I add a few scattered remarks.
After Einstein's brief flirtation with the Dirac equation, (Section 25b), he was
led to the belief that the sought-for equations of the total field would generate
particles with nonzero spin in terms of particle-like solutions that are not spher-
ically symmetrical (V. Bargmann, private communication). Presumably, he hoped
that his idea of overdetermination would lead to discrete spin values.*
He also hoped that the future theory would contain solutions which would not
be absolutely localizable and which would correspond to particles carrying quan-
tized electric charge [E4].
In 1925 Einstein noted that if the combined gravitational-electromagnetic field
equations have particle-like solutions with charge e and mass m, then there should
also be solutions with ( — e,m)\ [E33]. The proof involves the application of time
reversal to the combined equations. (In a related context, the existence of (+ e,m)
solutions was first noted by Pauli [PI].) This result led him to doubt temporarily
whether the unification of gravitation and electromagnetism was possible at all.
(Remember his demand that the unified field theory should generate the known
particles as special solutions.)
Simplicity was the guide in Einstein's quest. 'In my opinion, there is the correct
path and ... it is in our power to find it. Our experience up to date justifies us in
*I note in passing that in 1925 Einstein gave a helping hand to Uhlenbeck and Goudsmit in the
explanation of the origins of the spin-orbit coupling of electrons in atoms [Ul].